In a communications system, techniques must be implemented for combining and distributing high frequency signals among various components. For example, in a system which receives high frequency communications signals through more than one antenna, the received signals must be combined in order to form a single signal. In a transmitter which uses more than one antenna, signals from a single source must be split into more than one signal in order for the signals to be present at the transmit antennas. A transmitter can also combine signals from several low power devices to form a high power signal for transmission through a single antenna.
At high frequencies, especially at microwave and millimeter wave frequencies, hybrid circuits are used in order to perform power combining and power splitting functions. Traditional branchline hybrids have a disadvantage in that they are asymmetric. In other words, the signal paths through the hybrid combiner or splitter are of unequal length. Thus, any losses which occur while signals are traveling through the hybrid power combiner or power splitter will be unequal. Therefore, signals cannot be split into essentially equal components. The problem is further complicated in that unequal power splitting also results in unequal frequency response, which results in amplitude imbalance at the edges of the operating band. This can be especially problematic when uniform signal magnitude is required at the outputs of a power splitter. Additionally, when used as a combiner, the loss of an input signal does not result in a predictable power output from the combiner structure.
A further drawback of a traditional asymmetric branchline hybrid splitter is that this type of structure produces outputs which have a quadrature phase relationship to each other. In many applications this is undesirable since additional phase shifting components must be added to compensate for the quadrature phase relationship between the signal outputs.
Waveguide magic tee structures are one option for producing in-phase power splitting or combining. However, a waveguide solution is often undesirable due to the size of the constituent wave guide components. Additionally, waveguide structures are inherently three dimensional and more costly to produce than corresponding microstrip and stripline approaches. Other structures exist for producing in-phase power combination and splitting such as the rat race or ring hybrid. However, these structures are also asymmetric and prone to undesirable coupling between input and output lines.
Another structure which can provide in-phase power combining and splitting is a Wilkinson hybrid. However, a Wilkinson hybrid requires the use of a lumped element resistor which functions as a circuit element. Therefore, as the physical length of the resistor approaches a quarter wavelength at the operating frequency, the performance of the hybrid is degraded. As the design frequency increases, any losses introduced by the physical length of the resistor become larger and larger, making the device unusable at millimeter wave frequencies. In addition to these limitations, a Wilkinson hybrid provides power splitting and power combining over a limited bandwidth. Although this bandwidth can be improved by using multiple sections, this increases the size required to implement the power combining and power splitting functions.
Therefore, what is needed is a power combiner and splitter which can be used over a greater bandwidth and provide in-phase power combining and power splitting.